So, I have found tonnes of information here and elsewhere about having to flash my junkyard PCM to work in my truck. However, I would like to stick to swapping parts for now till I have the monies to afford a top notch programmer.

Back story: I swapped axle ratios from 3:42 to 3:73. I need to swap the PCM to get my speedo corrected (again). I found another 98 truck exactly like mine in the junkyard but this truck had a 3:73. The service numbers do not match, I assume due to the different ratios. I checked the RPO codes and they were pretty close to having the same options.

My question: Instead of buying tuning software or paying GM (definitely not going to pay GM). Is it possible to swap both the PCM and the BCM in order for the new system to pass the vin code check? If this is possible in anyway, are there other modules that must be swapped?

 

Much easier to find someone to change the gear ratio in the pcm you have.

 

I would like to stick to swapping parts until I can make the change myself. I have priced tunes and seems like $150 is the lowest the service is offered. I have already committed to the junkyard PCM and they are one of those no cash back places.
I have pinged all of the available modules from the OBD2 port with my Tech2 and only the passlock module, BCM and antilock brake controller are available. So I think that narrows it down a bit, but still not sure of which one is the ticket.

 

For those of you who are not down for getting a PCM tune, or have a junk yard where you get things on the cheap. I changed the PCM in my truck without getting a tune to disable VATs. Cost me roughly $70 at the scrap yard for the PCM and passlock module. This worked on my 98 K1500.
The instructions are simple, ensure the RPO codes in the glove box match what you want/need, in my case 3.42 stock to 3.73 current rear end ratio. Pull the PCM and the passlock module. The passlock module is a small plastic box that is hidden under the auxiliary CD player. Pull the CD player and the four plastic tabs are visible.
Next step is really easy, walk out of the yard with parts in hand.
When you go to install these in your truck, disconnect the battery, pull the old and install the new. The passlock is a little tricky, but not too bad to get back into position. The next step is time consuming, you need to perform an auto relearn so your new passlock can be reprogrammed to accept your trucks passlock key (the resistor in the key cylinder). When you turn the key to the on position observe the security light, it should be flashing. Bump the starter for a second then let the key in the run position for 10 minutes, or until the security light stops flashing. Do this three times, totalling 30 minutes. After the third time the trucks PCM accepts the token from the passlock and all is well. I have put over 100 miles on the new PCM and I have had zero issues.

Forgive any grammatical errors, working from a cell phone.